Nucleolar organizer regions (NORs), claimed to represent valuable markers of tumor proliferation, were visualized on paraffin sections by an argyrophil method and counted in 18 nontumorous adenohypophyses and 132 pituitary tumors. The AgNOR counts could not be correlated with the hormonal activity of nontumorous and adenoma cells. In pituitary adenomas, the mean AgNOR values were higher than in their corresponding nontumorous cell types. Some adenomas, especially growth hormone (GH), and prolactin (PRL) cell adenomas, however, had AgNOR readings in the range of nontumorous cells. Long-acting somatostatin analog and bromocriptine treatment decreased AgNOR counts in GR- and PRL-producing tumors. Most, but not all invasive and/or recurrent adenomas had high AgNOR counts. In a corticotroph carcinoma, AgNORs were not higher than in the adenomas. These inconsistent results limit, at the present time, the use of AgNORs as reliable markers of cell proliferation in pituitary tumors. Further studies may help to establish the value of this promising method in pituitary pathology.